Unix Power Tools, 3rd Edition

Chapter 12. Showing What’s in a File

Cracking the Nut

Summary Box

This chapter talks about the many ways of dumping a file to the screen. Most users know the brute force approach provided by cat (Section 12.2), but there’s more to it than that:

• Pagers such as more and less (Section 12.3) that give you more control when looking through long files.

• Finding out what type of data a file contains before opening it (Section 12.6).

• Looking at just the beginning or end of a file (Section 12.8 through Section 12.12).

• Numbering lines (Section 12.13).

— TOR

What Good Is a cat?

The cat command may well be the first command new users hear about, if only because of its odd name. cat stands for concatenate or, as some would say, catenate. Both words mean the same thing: to connect in a series. The cat command takes its filename arguments and strings their contents together. Essentially, cat takes its input and spits it out again.

cat has many uses, but the four most basic applications are described in the following list. In many ways, they don’t illustrate cat so much as they illustrate the shell’s output redirection (Section 43.1) mechanism.

1. First form:

   % cat 
                        file
   % cat 
                        file1 file2 file
                        ...

   Use this form to display one or more files on the screen. The output doesn’t pause when the screen is full. As a result, if your files are more than one screenful long, the output will whiz by without giving you a chance to read it. To read output by screenfuls, use a pager such as less (Section 12.3).^[1]

2. Second form: ...